Method of making a plasma lamp

ABSTRACT

An apparatus and method for achieving desired spectral emission characteristics in plasma lamps is disclosed. The apparatus and method use multi-layer thin film optical interference coatings to selectively reflect a portion of the light such that it can be absorbed in the plasma. The multi-layer thin film coating is applied to any surface of the lamp, which substantially surrounds the plasma. The number and thickness of the layers in the coating are selected to ensure that significant portion of the selected light emitted from the plasma is reflected by the coating and absorbed by the plasma. The properties of the coating, reflectance, transmittance and absorption are determined as a function of plasma and lamp characteristics. These characteristics include the spectral emission characteristics of the plasma, the spectral absorption characteristics of the plasma, the physical dimensions of the plasma, the angular distribution of the light emitted from the plasma on the coating and the geometry of the coated surface.

RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.10/112,024, filed Apr. 1, 2002, now U.S. Pat. No. 6,897,609, whichclaims the benefit of U.S. Provisional Patent Application No.60/279,685, filed Mar. 30, 2001, each of which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

The present invention generally relates to electric lamps and methods ofmanufacture. More specifically, the present invention relates to lampswherein the light source includes a light emitting plasma containedwithin an arc tube (i.e. plasma lamps) having dichroic thin filmcoatings to improve the operating characteristics of the lamp.

Plasma lamps such as mercury lamps or metal halide lamps have foundwidespread acceptance in lighting large outdoor and indoor areas such asathletic stadiums, gymnasiums, warehouses, parking facilities, and thelike, because of the relatively high efficiency, compact size, and lowmaintenance of plasma lamps when compared to other lamp types. A typicalplasma lamp includes an arc tube forming a chamber with a pair of spacedapart electrodes. The chamber typically contains a fill gas, mercury,and other material such as one or more metal halides, which arevaporized during operation of the lamp to form a light emitting plasma.The operating characteristics of the lamp such as spectral emission,lumens per watt (“LPW”), correlated color temperature (“CCT”), and colorrendering index (“CRI”) are determined at least in part by the contentof the lamp fill material.

The use of plasma lamps for some applications has been limited due thedifficulty in realizing the desired spectral emission characteristics ofthe light emitting plasma. For example, metal halide lamps wereintroduced in the United States in the early 1960's and have been usedsuccessfully in many commercial and industrial applications because ofthe high efficiency and long life of such lamps compared to other lightsources. However, metal halide lamps have not as yet found widespreaduse in general interior retail and display lighting applications becauseof the difficulty in obtaining a spectral emission from such lampswithin the desired range of CCT of about 300

-400

K and CRI of greater than about 80.

Relatively high CRI (>80) has been realized in metal halide lamps havinga CCT in the desired range by the selection of various metal halidecombinations comprising the lamp fill material. For example, U.S. Pat.No. 5,694,002 to Krasko et al. discloses a metal halide lamp having aquartz arc tube with a fill of halides of sodium, scandium, lithium, andrare earth metals, which operates at a CCT of about 300

K and a CRI of about 85. U.S. Pat. No. 5,751,111 to Stoffels et al.discloses a metal halide lamp having a ceramic arc tube with a fill ofhalides of sodium, thallium and rare earth metals which operates at aCCT of about 300

K and a CRI of about 82. However, the quartz lamps disclosed by Kraskoet al. have a relatively low LPW, the ceramic lamps disclosed byStoffels et al. are relatively expensive to produce, and both types oflamps have a relatively high variability in operating parameters and arelatively diminished useful operating life.

The use of a sodium/scandium based halide fill in plasma lamps hasaddressed the efficiency and variability problems by providing improvedefficiency and lower variability in operating parameters relative tometal halide lamps having other fill materials. However, such lamps havea relatively low CRI of about 65-70 and thus are not suitable for manyapplications.

One known approach in improving certain operating characteristics ofplasma lamps is to filter the light emitted from the plasma. Recentdevelopments in thin film coating technology have increased the utilityof such coatings in the lighting industry by improving both the thermalcapability of the coatings and the uniformity of such coatings whenapplied to curved surfaces such as the arc tubes, reflectors, and outerenvelopes of lamps. The MicroDyn® reactive sputtering process ofDeposition Sciences, Inc. of Santa Rosa, Calif., as disclosed andclaimed for example in U.S. Pat. No. 5,849,162 is particularly suitablefor depositing a variety of thin film coatings useful in lightingapplications. Other known coating processes such as chemical vapordeposition, thermal evaporation, and ion and electron beam depositionmay also be suitable for lighting applications.

It is a characteristic of such coatings that they selectively reflectand/or absorb radiation at selected wavelengths. For example, U.S. Pat.No. 5,552,671 to Parham et al. discloses a multilayer UV radiationabsorbing coating on the arc tubes of metal halide lamps to block UVradiation. U.S. Pat. No. 5,646,472 to Horikoshi discloses a metal halidelamp having a dysprosium based fill with a multilayer coating on the arctube for reflecting light at wavelengths shorter than nearly 600 nmwhile transmitting light at longer wavelengths to lower the CCT of thelamp. However, the optimal utilization of thin film coatings to controlcertain operating characteristics of plasma lamps often requires that asignificant portion of the light that is selectively reflected by thecoating be absorbed by the plasma, and there remains a need for thinfilm coatings for plasma lamps directed to plasma absorption.

It is accordingly an object of the present invention to obviate many ofthe deficiencies of the prior art and to specifically address the plasmaabsorption of reflected light in the improvement of the operatingcharacteristics of plasma lamps.

Another object of the present invention is to improve the effectivenessof thin film coatings used in plasma lamps by consideration of theabsorption of reflected light in the plasma in the design andfabrication of such coatings.

Still another object of the present invention is to provide a novelmultilayer thin film filter and method for plasma lamps.

Yet another object of the present invention is to provide a novel plasmalamp with improved operating characteristics and method of manufacturingsuch plasma lamps.

Still yet another object of the present invention to provide a novelplasma lamp and method using multilayer thin film coatings to obtain thedesired spectral emission characteristics for the lamp.

A further object of the present invention is to provide a novel plasmalamp and method of making plasma lamp with operating characteristicssuitable for indoor retail and display lighting.

Yet a further object of the present invention to provide a novel metalhalide lamp and method having a highly selective notch intransmissivity.

Still a further object of the present invention to provide a novelmethod of making multilayer thin film coatings for plasma lamps whereinthe number and thickness of the layers in the coating are determined asa function of the spectral and/or physical characteristics of theplasma.

Yet still a further object of the present invention to provide a novelmethod of making multilayer thin film coatings for plasma lamps whereinthe number and thickness of the layers in the coating are determined asa function of the geometry of the surface to be coated and/or andangular distribution of the light emitted from the plasma on thecoating.

It is still another object of the present invention to provide a novelsodium/scandium lamp and method.

These and many other objects and advantages of the present inventionwill be readily apparent to one skilled in the art to which theinvention pertains from a perusal of the claims, the appended drawings,and the following detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a formed body arc tube for plasma lamps.

FIG. 2 is an illustration of the transmissivity characteristics of amultilayer coating according to one aspect of the present invention.

FIG. 3 is an illustration of the variability of the CRI of the lighttransmitted by filters as a function of the location of the filtercenter.

FIG. 4 is an illustration of the variability of the CRI and CCT versusLPW reduction of a sodium/scandium metal halide lamp having an arc tubewith a multilayer coating according to one aspect of the presentinvention.

FIG. 5 a illustrates the transmissivity characteristics of a coatingaccording to another aspect of the present invention.

FIGS. 5 b and 5 c illustrate the spectral emission from a mercury lampwith no filter and with the filter of FIG. 5 a respectively.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention finds utility in the manufacture of all types andsizes of plasma lamps. As discussed above, plasma lamps have foundwidespread acceptance in many lighting applications, but the use ofplasma lamps in some applications may be limited due to the difficultyin realizing the desired spectral emission characteristics of the lightemitting plasma in such lamps. It has been discovered that multilayerthin film optical interference coatings designed so that a significantportion of the light that is selectively reflected by the coating isabsorbed by the plasma provide a means for obtaining the desiredspectral emission characteristics while maintaining or improving theoverall operating characteristics of plasma. By way of example only,certain aspects of the present invention will be described in connectionwith obtaining the desired spectral emission characteristics insodium/scandium metal halide lamps to raise the CRI of such lamps.

FIG. 1 illustrates a formed body arc tube suitable for use insodium/scandium metal halide lamps. With reference to FIG. 1, the arctube 10 is formed from light transmissive material such as quartz. Thearc tube 10 forms a bulbous chamber 12 intermediate pinched end portions14. A pair of spaced apart electrodes 16 are sealed in the arc tube, onein each of the pinched end portions 14. The chamber 12 contains a fillgas, mercury, and one or more metal halides.

During operation of the lamp, an arc is struck between the electrodes 16that vaporizes the fill materials to form a light emitting plasma.According to the present invention, a multilayer thin film coating maybe applied to any surface in the lamp which substantially surrounds theplasma, e.g., the arc tube, an arc tube shroud, the outer lamp envelope,or a reflector. According to certain aspects of the present invention,the number and thickness of the layers comprising the coating aredetermined so that a significant portion of the light emitted from theplasma that is selectively reflected by the coating is absorbed in theplasma. In the coatings of the present invention directed to plasmaabsorption, the properties of the coating (including reflectance,transmittance, and absorption) are determined as a function of severalplasma and lamp characteristics including the spectral emissioncharacteristics of the plasma, the spectral absorption characteristicsof the plasma, the physical dimensions of the plasma, the angulardistribution of the light emitted from the plasma on the coating, andthe geometry of the coated surface.

To obtain a desired spectral emission from a plasma lamp using a filter,the target spectral emission lines must be identified by analysis of theunfiltered spectral emission of the lamp. The filter must then bedesigned so that desired portions of the light emitted by the plasma atthe target wavelengths are reflected by the filter and absorbed in theplasma to thereby selectively remove such light from the lighttransmitted from the lamp.

Once the target spectral lines have been identified, the physicaldimensions of the specific arc in the plasma that primarily emit thelight at each targeted wavelength are measured to determine the regionwithin the plasma that the reflected light must be directed forabsorption.

The spectral absorption characteristics of the plasma are thendetermined either theoretically by consideration of arc temperature andthe densities of the mercury and metal halides, or experimentally basedon measured spectral emittance changes caused by the application ofhighly reflective coatings to the arc tube.

The angular distribution of the light emitted from the plasma on thefilter must also be determined so that the angle of incidence may beconsidered in the coating design. The geometry of the filter (i.e. thecoated surface), and the physical dimensions of the plasma may be usedto determine the angular distribution of the emitted light at each pointon the filter.

In view of the dimensions of the plasma and the angular distribution ofthe emitted light on the filter, the absorption of light in the plasmaas a function of the reflectivity of the filter may be predicted.

The reflectivity levels at each spectral emission wavelength of interestfor the filter may then be targeted to obtain the desired spectraltransmission from the lamp. The number and thickness of the layerscomprising the multilayer coating may then be determined usingtechniques that are common in the thin film coating art to obtain acoating having the desired properties.

The coating may be deposited using any suitable deposition process suchas reactive sputtering, chemical vapor deposition, thermal evaporation,and ion or electron beam deposition. A suitable multilayer coatingtypically includes alternating layers of materials having differingindices of refraction.

A typical sodium/scandium metal halide lamps includes a fill comprisinga fill gas selected from the gases neon, argon, krypton, or acombination thereof, mercury, and halides of sodium and scandium. Thefill material may also include one or more additional halides of metalssuch as thorium and metals such as scandium and cadmium.

In the aspect of the present invention directed to raising the CRI ofsodium/scandium metal halide lamps, based on an analysis of the spectralemission of such lamps, it has been determined that the CRI of the lighttransmitted by a notch filter that reflects at least seventy percent ofthe light emitted by the plasma in a narrow wavelength band (about 550nm to about 620 nm) in the visible spectrum (about 380 nm to about 760nm) and transmits at least seventy percent of the light emitted from theplasma in the visible spectrum and outside of the narrow band is greaterthan the CRI of the light emitted from the plasma. (Note that thepercentages of light transmitted or reflected relate to the averagetransmission/reflection of light within the identified band and not thespecific transmission/reflection of light at each wavelength in theband.) A suitable coating may comprise alternating layers of silica (theL material) and an oxide of zirconium, tantalum, titanium, niobium, orhafnium (the H material). The overall thickness of the coating may be3-10 microns with the thickness of individual layers ranging between0.1-2000 nm.

Table I illustrates the composition of a multilayer coating applied tothe outer surface of the arc tube of a typical sodium/scandium lamp(unfiltered CRI 65-70) according to the present invention.

TABLE I Layer composition and thickness for a 78-layer film of ZrO2/SiO2LAYER MATERIAL THICKNESS (nm) 1 ZrO₂ 25.39 2 SiO₂ 31.03 3 ZrO₂ 41.69 4SiO₂ 29.96 5 ZrO₂ 57.27 6 SiO₂ 29.8 7 ZrO₂ 32.24 8 SiO₂ 31.3 9 ZrO₂72.39 10 SiO₂ 30.66 11 ZrO₂ 29.48 12 SiO₂ 30.76 13 ZrO₂ 68.5 14 SiO₂30.78 15 ZrO₂ 28.04 16 SiO₂ 30.5 17 ZrO₂ 64.69 18 SiO₂ 30.64 19 ZrO₂24.31 20 SiO₂ 30.52 21 ZrO₂ 64.17 22 SiO₂ 30.43 23 ZrO₂ 23.73 24 SiO₂30.78 25 ZrO₂ 66.68 26 SiO₂ 30.85 27 ZrO₂ 25.71 28 SiO₂ 30.51 29 ZrO₂66.4 30 SiO₂ 30.71 31 ZrO₂ 25.13 32 SiO₂ 30.47 33 ZrO₂ 67.99 34 SiO₂30.46 35 ZrO₂ 24 36 SiO₂ 30.93 37 ZrO₂ 69.53 38 SiO₂ 30.85 39 ZrO₂ 22.6440 SiO₂ 30.61 41 ZrO₂ 67.84 42 SiO₂ 30.72 43 ZrO₂ 23.35 44 SiO₂ 30.43 45ZrO₂ 66.43 46 SiO₂ 30.37 47 ZrO₂ 25.34 48 SiO₂ 30.91 49 ZrO₂ 67.61 50SiO₂ 30.77 51 ZrO₂ 25.36 52 SiO₂ 30.57 53 ZrO₂ 66.58 54 SiO₂ 30.74 55ZrO₂ 24.96 56 SiO₂ 30.41 57 ZrO₂ 63.75 58 SiO₂ 30.35 59 ZrO₂ 26.97 60SiO₂ 30.85 61 ZrO₂ 68.31 62 SiO₂ 30.71 63 ZrO₂ 28.83 64 SiO₂ 30.69 65ZrO₂ 72.26 66 SiO₂ 31.23 67 ZrO₂ 32.68 68 SiO₂ 29.87 69 ZrO₂ 58.29 70SiO₂ 30.1 71 ZrO₂ 42.63 72 SiO₂ 30.99 73 ZrO₂ 25.26 74 SiO₂ 1020.87 75ZrO₂ 21.46 76 SiO₂ 21.34 77 ZrO₂ 121.69 78 SiO₂ 99.84

As illustrated, the coating disclosed in table I includes alternatinglayers of SiO2 and ZrO2 and 78 total layers. FIG. 2 illustrates thetransmissivity of the coating disclosed in Table I. As illustrated, thecoating forms a notch filter that reflects nearly all of the incidentlight in a narrow band substantially centered on a wavelength of about590 nm, and transmits nearly eighty percent of the incident light in thevisible spectrum and outside of the narrow band. A 400 wattsodium/scandium lamp with the multilayer coating of Table I applied tothe outer surface of the arc tube operates at a CCT of 400

K with a CRI of 85 and a LPW of 85.

Thus according to one aspect of the present invention, the CRI of asodium/scandium lamp may be raised by 15-20 points while maintaining arelatively efficient lamp.

It has been discovered that a CRI of greater than 90 may be realized ina sodium/scandium lamp depending on the location of the reflected bandin the visible spectrum as illustrated in FIG. 3. However, improvementsin CRI must be obtained with consideration of any loss in lumen outputof the lamp. FIG. 4 illustrates the variability of the CRI and CCTversus LPW reduction of a 400 watt sodium/scandium metal halide lamphaving an arc tube with a multilayer coating according to one aspect ofthe present invention.

In another aspect of the present invention, a multilayer coating may beused in a mercury lamp to reduce the transmission of light emitted at405 nm and 435 nm to thereby selectively alter the emission spectrum ofthe lamp. By eliminating emission at wavelengths that are useless ordetrimental for an application, the energy efficiency of the lamp can beimproved.

Table II illustrates the composition of a multilayer coating applied tothe outer surface of the arc tube of a typical mercury lamp according tothe present invention.

TABLE II Layer composition and thickness for a 15-layer film ofZrO2/SiO2 LAYER MATERIAL THICKNESS (nm) 1 ZRO2 17.65 2 SIO2 107.71 3ZRO2 35.30 4 SIO2 107.71 5 ZRO2 35.30 6 SIO2 107.71 7 ZRO2 35.30 8 SIO2107.71 9 ZRO2 35.30 10 SIO2 107.71 11 ZRO2 35.30 12 SIO2 107.71 13 ZRO235.30 14 SIO2 107.71 15 ZRO2 17.65

As illustrated, the coating disclosed in Table II includes alternatinglayers of SiO2 and ZrO2 and 15 total layers. FIG. 5 a illustrates thetransmissivity of the coating disclosed in Table II. As illustrated, thecoating reflects nearly all of the incident light at the targetedspectral lines of 405 nm and 435 nm. FIG. 5 b illustrates the unfilteredspectral emission from a mercury lamp. FIG. 5 c illustrates the spectralemission from the mercury lamp of FIG. 5 b with the multilayer coatingof table II applied to the arc tube.

The multilayer coatings of the present invention find utility inimproving a wide range of operating characteristics in plasma lamps. Asdisclosed by way of example, the a multilayer coating may be used toimprove the CRI of a sodium/scandium lamp or selectively alter theemission spectrum and/or improve the energy efficiency of a mercurylamp. Other advantages in the operating characteristics of such lampsmay also be realized by the effects of the coatings on parameters suchas the temperature of the arc tube wall, the halide pool distribution,the size and shape of the plasma, and the infrared emission from thelamp.

While preferred embodiments of the present invention have beendescribed, it is to be understood that the embodiments described areillustrative only and the scope of the invention is to be defined solelyby the appended claims when accorded a full range of equivalence, manyvariations and modifications naturally occurring to those of skill inthe art from a perusal hereof.

1. A method of making a high intensity discharge lamp having avaporizable fill material of one or more metal halides forming a lightemitting plasma during operation of the lamp, said method comprising thesteps of: selecting a fill material comprising halides of sodium,scandium and thorium; and filtering the light emitted from the plasma,so that the operating characteristics of said lamp include a lumens perwatt greater than about 85, a color rendering index greater than about80, and a correlated color temperature between about 3000° K. and about6000° K.
 2. The method of claim 1 wherein the step of filtering thelight comprises providing a notch filter which reflects at least seventypercent of the light generated by the lamp within a narrow wavelengthband in the visible spectrum and transmits at least seventy percent ofthe light generated by the lamp within the visible spectrum and outsideof said narrow band.
 3. The method of claim 2 wherein the notch filterreflects at least eighty percent of the light generated by the lampwithin a narrow wavelength band in the visible spectrum and transmits atleast eighty percent of the light generated by the lamp within thevisible spectrum and outside of said narrow band.
 4. The method of claim2 wherein the narrow wavelength band is substantially centered on awavelength of about 590 nm.
 5. A method of making a lamp comprising thesteps of: (a) providing an arc tube containing a light emitting plasma;(b) determining the spectral emission characteristics of the plasma; (c)identifying wavelengths of light undesirable for transmission from thelamp; (d) determining the spectral absorption characteristics of theplasma; (e) identifying respective regions within the plasma efficientin absorbing light at each of the identified wavelengths; and (f)providing a filter on the arc tube that substantially reflects theplasma-emitted light at the identified wavelengths back towards theidentified respective regions.
 6. The method of claim 5 wherein the stepof providing a filter comprises the steps of determining the number andthickness of the layers in a multilayer coating and applying the coatingto a surface of the arc tube.
 7. A method of making a plasma lampcomprising: (a) determining the spectral emission from the plasma; (b)determining the location in the plasma of one or more arcs emittinglight at one or more wavelengths of interest; (c) determining the angleof incidence to the arc tube of the light emitted at one or more of thewavelengths of interest; (d) determining the number and thickness of thelayers of a multilayer coating for application to the arc tube so thatat least a portion of the light emitted from the plasma at the one ormore of the wavelengths of interest is reflected by the coating towardthe arc emitting the light at the wavelength of interest; and (e)applying the multilayer coating to the arc tube.
 8. The method of claim7 wherein the CRI of the light transmitted by the coating is greaterthan about 80.